Such are the demands of celebrity today that it's no longer enough for someone to be just an actor or a singer. They have to be both, and ideally have another sideline as a model, rapper, perfume entrepreneur or fashion mogul. Baseball has a great phrase for this kind of multitasking: such people are known as "triple threats". In baseball, though, triple threats are rare gems: men who can catch, hit and run brilliantly, making them the perfect player. It's not quite the same when the words are used to describe someone who has a TV show, a perfume line and an "eclectic album that shows off their many sides" on the go.

Of course, entertainment triple threats aren't a new concept. Think of Frank Sinatra, Judy Garland or Bing Crosby - all of them could dance like a dream, sing with panache and command the cinema screen with their acting chops. No doubt 50 Cent thinks he is following in this fine tradition now that he has made his acting debut, even though his performance in Get Rich Or Die Tryin' isn't going to cause Denzel Washington any sleepless nights. But it's time for these media sensations to get a reality check. Being a triple threat means excelling in three different arenas, not clogging up the charts, cinemas and television with work that is at best competent, but a lot of the time bland, revealing nothing more than a lack of talent.

For some celebrities, the very attempt to become a triple threat can tarnish the real talent that brought fame in the first place. Jennifer Lopez illustrates this perfectly. She delivered some excellent performances in the movies Out of Sight and Selina, but hasn't made a film to rival them since. Instead she underwent an image metamorphosis, transforming herself into the brand J Lo, and released an irritating single assuring us that she is still Jenny from the block. Lopez is clearly a canny businesswoman who has seized upon the idea of marketing herself as a kind of entertainment one-stop shop - but anyone who comes inside to browse can only leave disappointed. Lopez is still making films, playing a series of downtrodden-women-done-good roles (Maid in Manhattan, The Wedding Planner, Monster-in-Law) that nicely feed into the J Lo brand, but none are exactly what you would call career highlights and it's hard to imagine her having one again.

Beyoncé Knowles is going to have to be careful if she's going to avoid a similar fate. As a singer, she is outstanding, and her live shows prove that she has the kind of stage presence that Tina Turner would envy. But is that enough to make her a decent movie actor? She was fine in Austin Powers: Goldmember, where she had very little to do except stand smothered in varying shades of gold and play up to the moniker, Foxxy Cleopatra. But her stage presence didn't exactly translate to the big screen. Playing opposite Cuba Gooding Jr in Fighting Temptations, Beyoncé made much of the fact that she had put on at least 20lb to play the role of a small-town singer. Well, she sure showed Robert De Niro. Will her starring role in the new Pink Panther remake as, what else, a glamorous entertainer called Xania make me eat my words? Not likely.

The trouble is, all these wannabe triple threats are more concerned with building up their brand than with testing their artistry and talent. Halle Berry was nothing more than a pretty ex-model who had dabbled in television when she took the role as a crack whore in Spike Lee's Jungle Fever - but that performance gave her instant big screen credibility and proved she was a good actor. What would Beyoncé's music company say if she suddenly showed enthusiasm for a similar script? There would have to be serious discussions about how such film roles impact on public perception and, by extension, her ability to hawk cosmetics or hair-care products.

This isn't to say that artists should be locked for ever in one field. We want to be amazed by our favourite stars, by the discovery that there are hidden depths to their talent pool. But surely this can be done with the minimum fanfare. It's one thing for Joaquin Phoenix to reveal that he can also sing (albeit like Johnny Cash), but quite another when model and actor Lindsay Lohan decides to inflict an album on us. And when an upstart like Nick Cannon, the presenter from MTV's Wild 'N Out, is billed as a "comedian", "rapper", "actor" and "executive producer" something has gone very, very wrong.